The invention relates to an LED array having a plurality of LED chains that each have at least one LED and are connected in parallel. The invention also relates to an LED module.
In operation, such LED arrays are usually connected to a current source that supplies the LED array with an operating current, the operating current and an operating voltage are fixedly prescribed.
For an LED array, a lower limit is often specified for the radiation generated, which lower limit must not be undershot. This applies particularly in the case of uses in signaling installations, such as, for example, traffic lights or rail signals. For reasons of traffic safety, particular consideration must be given here to complying with the prescribed lower limits, which generally relate to the light power.
On account of aging phenomena, the radiation yield decreases as the operating duration increases in the case of LEDs and hence in the case of LED arrays of the above-mentioned type. In order to ensure that predetermined lower limits are complied with throughout the service life of an LED array, over dimensioning of the LED array or operation with an increased operating voltage is necessary, by way of example. These measures are in each case to be dimensioned such that even in the worst-case scenario, toward the end of the service life, a sufficient light power above the predetermined lower limit is achieved. It is accepted in this case that significantly more light than would be necessary for the envisaged function of the LED array is generated at the beginning of operation. Moreover, the production costs of an LED array rise due to the over dimensioning thereof, and an increased operating voltage leads to higher operating costs and possibly to accelerated aging.
It is accordingly an object of the invention to provide a LED array and a LED module that overcome the above-mentioned disadvantages of the prior art devices of this general type, which has improved driving. In particular, it is an object of the invention to provide an LED array that has a radiation power that is as constant as possible even during long-term operation. with the foregoing and other objects in view there is provided, in accordance with the invention, an LED array. The LED array contains a plurality of LED chains each having at least one LED and connected in parallel with each other, and at least one output for feeding back radiation generated to a power supply unit.
The invention is based on the idea of providing, in the LED array, an output via which optical parameters of the LED array, such as the radiation power, for example, can be fed back to a power supply unit.
According to the invention, the LED array contains a plurality of LED chains each having at least one LED, which are connected in parallel. The LED array has at least one output for a feedback of the radiation power generated to the power supply unit.
In this case, a reference LED chain having at least one LED is preferably provided, the optical parameters of which, for example the radiation power, are detected by a photosensitive component and converted into an electrical measurement signal, which is available at the output for feedback to the power supply unit.
By the power supply unit, the operating voltage or the operating current can be set using the measurement signal such that a largely constant radiation power is achieved and, in particular, a predetermined lower limit for the radiation power is not undershot.
Overall, an optimum modulation of the LED array is thus produced.
It is advantageous that such an LED array does not have to be operated far above the minimum requirements at the beginning of operation. As a result, a lower degree of energization is produced from the start, which leads to a longer service life of the LED array. Moreover, the operation of the module is more cost-effective on account of the lower electrical power demand.
The tracking of the energization in accordance with the optical parameters such as the radiation power, for example, ensures that the optical minimum requirements are complied with throughout the service life significantly more reliably than in the case of conventional LED arrays with fixedly prescribed values for the operating voltage or the operating current.
In the case of the invention, the LEDs are preferably surface-mountable LEDs, which enable LED modules with a high packing density and a correspondingly high luminance.
In an advantageous refinement of the invention, the reference LED chain contains a series circuit of a plurality of LEDs. A reference LED chain with a plurality of LEDs has the advantage that the aging behavior of the reference LEDs as a whole comes closer to the aging behavior of the LED module than an individual LED. In particular, statistical fluctuations of the LED parameters and manufacturing tolerances of the LEDs are compensated for better in this way.
Furthermore, it is advantageous to configure the LED array such that the photosensitive component detects only the radiation generated by the reference LED chain. Different functions are thus assigned to the LED chains, on the one hand, and the reference LED chain, on the other hand. The reference LED chain serves essentially, preferably exclusively, for monitoring the optical parameters, while the other LED chains serve essentially, preferably exclusively, for radiation generation, for example for illumination or signaling purposes. This subdivision of the functions has the advantage that it is not necessary to monitor the radiation generation in all of the LED chains simultaneously, which can lead to a considerable technical outlay in the case of relatively large LED arrays. In particular, the reference LED chain together with the photosensitive component can be separated from the other LED chains, since radiation toward the outside is neither necessary nor desirable in the case of the reference LED chain. Conversely, this results in that the monitoring of the optical parameters is advantageously not corrupted by externally incident extraneous light such as solar radiation, for example. If only the radiation generated by the LED array is detected overall, then it is generally necessary to take particular precautions that prevent such corruption by extraneous light.
The separation of the reference LED chain from the other LED chains can be achieved by virtue of the fact that the LED array is provided with an encapsulation with a radiation exit opening through which is emitted only the radiation of the LED chains, but not the radiation of the reference LED chain.
A further separation possibility consists in providing a common main radiating direction for the LED chains, which main radiating direction differs from a reference radiating direction assigned to the reference LED chain. By way of example, the LED chains, and the reference LED chain, can be disposed on different sides of a printed circuit board, so that the main radiating direction and the reference radiating direction point in opposite directions.
A screen is preferably provided for separating the reference LED chain from the other LED chains, which screen covers the reference LED chain together with the photosensitive component. This prevents, in particular, the detection of the radiation generated by the reference LED chain from being corrupted by radiation from the outside. In this case it is advantageous to provide that side of the screen that faces the reference LED chain with a diffusely reflecting surface or coating. This increases the radiation component available for detection. Furthermore, the averaging over the radiation components generated by the individual LEDs and, consequently, the compensation of the above-mentioned statistical fluctuations of the LED parameters and of the manufacturing tolerances are improved.
In a preferred refinement of the invention, the LED array contains a converter circuit, which is connected to the photosensitive component and generates the measurement signal. By way of example, a photodiode, in particular a PIN photodiode, may serve as the photosensitive component. The converter circuit generates a voltage in a manner dependent on the radiation detected by the photosensitive component, an output coupler, preferably an optocoupler, being driven by the voltage on the input side. The measurement signal for feedback to the power supply unit is then present at the output-side terminals of the output coupler. In the case of an optocoupler, by way of example, the resistance between the output-side terminals or a photocurrent generated in the optocoupler can be used as the measurement signal, depending on the embodiment.
In a development of this refinement, a photodiode is connected in series with a series resistor, so that a voltage that is proportional to the photocurrent that is generated by the photodiode and is dependent on the detected radiation is dropped across the series resistor. The voltage drives the base of a transistor whose emitter is connected to a first reference potential via a zener diode and whose collector is connected to a second reference potential via a collector resistor. The second reference potential is greater, in the case of an npn transistor, and less, in the case of a pnp transistor, than the first reference potential. Furthermore, the output coupler provided is an optocoupler whose input side, for example whose input-side LED, is connected in parallel with the collector-emitter path of the transistor.
The series resistor defines a threshold value for the radiation to be detected. Below the threshold value, the transistor is in the off state and a current determined by the collector resistor and the zener diode flows through the input side of the optocoupler. The threshold value is exceeded if the photocurrent generated by the photodiode is so large that the voltage dropped across the series resistor exceeds the sum of the zener voltage and the base-emitter voltage (the latter is typically about 0.65 V). A current then flows through the collector-emitter path of the transistor, i.e. the transistor partly short-circuits the input of the optocoupler, so that the current through the optocoupler is reduced. The output resistance of the optocoupler changes correspondingly. This change in resistance serves as a measurement signal for controlling the power supply unit.
In a further advantageous refinement of the invention, a regulating configuration for regulating a predetermined current distribution between the individual LED chains and the reference LED chain is in each case connected in series with each LED chain and with the reference LED chain. This advantageously avoids the situation where the currents in the individual LED chains deviate to an excessively great extent from the predetermined desired current intensities on account of different forward voltages of the individual LEDs or a short circuit of an LED.
Preferably, the regulating configurations each contain a current amplification circuit for impressing a predetermined operating current into the LED chains in accordance with the predetermined current distribution. In this case, the current amplification circuits may have regulating inputs for regulating the respective current in the LED chain, the regulating inputs preferably being connected to one another and being at the same potential.
Within the scope of the invention, an LED module is furthermore provided which contains the LED array according to the invention and also a power supply unit. The LED array has supply terminals to which the power supply unit is connected on the output side. The power supply unit has a control input for controlling the output-side operating voltage or the output-side operating current with which the LED array is supplied. In this case, the output of the LED array, at which the measurement signal for feedback of the radiation generated is present, is connected to the control input of the power supply unit. It is advantageous that, in the case of such an LED module, by the feedback of the radiation generated to the power supply unit, operating parameters such as operating current and operating voltage can be tracked in such a way that a virtually constant optical power is achieved over the entire service life or a decrease in the radiation efficiency in the case of the LED chains, which decrease occurs with advancing aging, is compensated for.
The invention is suitable in particular for use in signaling installations such as traffic lights or rail signaling installations. In this case, it is particularly advantageous that the invention ensures a virtually constant radiation power and thus that prescribed lower limits for the radiation power are complied with for the entire service life, without the need for over dimensioning or excessively increased radiation generation at the beginning of the service life.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a LED array and a LED module, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.